Cosmic history of integrated galactic stellar initial mass function : a simulation study (original) (raw)
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Cosmic History of the Integrated Galactic Stellar Initial Mass Function: A Simulation Study
The Astrophysical Journal, 2015
Theoretical as well as observational studies suggest that the stellar initial mass function (IMF) might become top heavy with increasing redshift. Embedded cluster mass function is a power law having index β, whose value still remains controversial. In the present work, we investigate the effect of evolving IMF and varying indices of β for the integrated galactic initial mass function, in relation to several measures of star formation rates of galaxies at various redshifts by random simulation. The resulting IGIMF is segmented power law at various redshifts having slopes α 1,IGIMF and α 2,IGIMF with a turnover at a characteristic mass m c ′. These differ from the stellar initial mass functions with slopes α 1,IMF , α 2,IMF , and characteristic masses m c for different values of redshift z, β, minimum and maximum masses of the embedded clusters.
Top-heavy integrated galactic stellar initial mass functions (IGIMFs) in starbursts
Arxiv preprint arXiv: …, 2010
Star formation rates (SFR) larger than 1000 M ⊙ yr −1 are observed in extreme star bursts. This leads to the formation of star clusters with masses > 10 6 M ⊙ in which crowding of the pre-stellar cores may lead to a change of the stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of ultra-compact dwarf galaxies and recent results on globular clusters suggest the IMF to become top-heavy with increasing star-forming density. We explore the implications of top-heavy IMFs in these very massive and compact systems for the integrated galactic initial mass function (IGIMF), which is the galaxy-wide IMF, in dependence of the star-formation rate of galaxies. The resulting IGIMFs can have slopes, α 3 , for stars more massive than about 1 M ⊙ between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass function (ECMF) slope (β) of 2.0, but only if the ECMF has no low-mass clusters in galaxies with major starbursts. Alternatively, β would have to decrease with increasing SFR > 10 M ⊙ yr −1 such that galaxies with major starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of observationally deduced IMF variations with redshift.
Top-heavy integrated galactic stellar initial mass functions in starbursts
Monthly Notices of the Royal Astronomical Society, 2010
Star formation rates (SFR) larger than 1000 M ⊙ yr −1 are observed in extreme star bursts. This leads to the formation of star clusters with masses > 10 6 M ⊙ in which crowding of the pre-stellar cores may lead to a change of the stellar initial mass function (IMF). Indeed, the large mass-to-light ratios of ultra-compact dwarf galaxies and recent results on globular clusters suggest the IMF to become top-heavy with increasing star-forming density. We explore the implications of top-heavy IMFs in these very massive and compact systems for the integrated galactic initial mass function (IGIMF), which is the galaxy-wide IMF, in dependence of the star-formation rate of galaxies. The resulting IGIMFs can have slopes, α 3 , for stars more massive than about 1 M ⊙ between 1.5 and the Salpeter slope of 2.3 for an embedded cluster mass function (ECMF) slope (β) of 2.0, but only if the ECMF has no low-mass clusters in galaxies with major starbursts. Alternatively, β would have to decrease with increasing SFR > 10 M ⊙ yr −1 such that galaxies with major starbursts have a top-heavy ECMF. The resulting IGIMFs are within the range of observationally deduced IMF variations with redshift.
Evolution of the High-Mass End of the Stellar Initial Mass Functions in Starburst Galaxies
The Astrophysical Journal, 2013
We investigate the time evolution and spatial variation of the stellar initial mass function (IMF) in star-forming disk galaxies by using chemodynamical simulations with an IMF model depending both on local densities and metallicities ([Fe/H]) of the interstellar medium (ISM). We find that the slope (α) of a powerlaw IMF (N(m) ∝ m −α) for stellar masses larger than 1M ⊙ evolves from the canonical Salpeter IMF (α ≈ 2.35) to be moderately top-heavy one (α ≈ 1.9) in the simulated disk galaxies with starbursts triggered by galaxy interaction. We also find that α in star-forming regions correlates with star formation rate densities (Σ SFR in units of M ⊙ yr −1 kpc −2). Feedback effects of Type Ia and II supernovae are found to prevent IMFs from being too top-heavy (α < 1.5). The simulation predicts α ≈ 0.23 log Σ SFR + 1.7 for log Σ SFR ≥ −2 (i.e., more topheavy in higher Σ SFR), which is reasonably consistent well with corresponding recent observational results. The present study also predicts that inner regions of starburst disk galaxies have smaller α thus are more top-heavy (dα/dR ∼ 0.07 kpc −1 for R ≤ 5 kpc). The predicted radial α gradient can be tested against future observational studies of the α variation in star-forming galaxies.
The Variation of Integrated Star Initial Mass Functions among Galaxies
Astrophysical Journal, 2005
The integrated galaxial initial mass function (IGIMF) is the relevant distribution function containing the information on the distribution of stellar remnants, the number of supernovae, and the chemical enrichment history of a galaxy. Since most stars form in embedded star clusters with different masses, the IGIMF becomes an integral of the assumed (universal or invariant) stellar IMF over the embedded
The Initial Mass Function of Massive Stars in Galaxies: Empirical Evidence
Symposium - International Astronomical Union, 1986
Observational constraints on the form of the high-mass stellar IMF are reviewed. The evidence includes star counts in the solar neighborhood, individual and composite star clusters, and nearby galaxies, and arguments based on integrated light and chemical evolution modeling. There is no convincing evidence for any systematic variations of the shape of the high-mass IMF. However, the various determinations are very uncertain, and do not allow any firm estimate of the logarithmic slope of the upper IMF; the appropriate value is somewhere between −1.3 and −2.3, with region-to-region variations smaller than about ±0.5. A number of lines of evidence suggest that the lower mass limit or mode mass of the IMF increases with increasing star formation rate, reaching perhaps 10–15 m⊙ in some starburst galaxies. It is also possible that the upper mass limit depends on metallicity, based on variations in excitation conditions of HII regions.
Arxiv preprint arXiv: …, 2011
The stellar initial mass function (IMF) describes the distribution in stellar masses produced from a burst of star formation. For more than fifty years, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the high-mass IMF slope for a sample of low-to-moderate redshift galaxies from the Galaxy And Mass Assembly survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMF-star formation rate dependency is discovered, which shows that highly star forming galaxies form proportionally more massive stars (they have IMFs with flatter power-law slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF.
Variability in the stellar initial mass function at low and high mass: three-component IMF models
Monthly Notices of the Royal Astronomical Society, 2004
Three-component models of the initial mass function (IMF) are made to consider possible origins for the observed relative variations in the numbers of brown dwarfs, solar-to-intermediatemass stars and high-mass stars. The differences between the IMFs observed for clusters, field and remote field are also discussed. Three distinct physical processes that should dominate the three stellar mass regimes are noted. The characteristic mass for most star formation is identified with the thermal Jeans mass in the molecular cloud core, and this presumably leads to the middle mass range by the usual collapse and accretion processes. Pre-stellar condensations (PSCs) observed in millimetre-wave continuum studies presumably form at this mass. Significantly smaller self-gravitating masses require much larger pressures and may arise following dynamical processes inside these PSCs, including disc formation, tight-cluster ejection, and photoevaporation as studied elsewhere, but also gravitational collapse of shocked gas in colliding PSCs. Significantly larger stellar masses form in relatively low abundance by normal cloud processes, possibly leading to steep IMFs in low-pressure field regions, but this mass range can be significantly extended in high-pressure cloud cores by gravitationally focused gas accretion on to PSCs and by the coalescence of PSCs. These models suggest that the observed variations in brown dwarf, solar-to-intermediate-mass and high-mass populations are the result of dynamical effects that depend on environmental density and velocity dispersion. They accommodate observations ranging from shallow IMFs in cluster cores to Salpeter IMFs in average clusters and whole galaxies to steep and even steeper IMFs in field and remote field regions. They also suggest how the top-heavy IMFs in some starburst clusters may originate and they explain bottom-heavy IMFs in low surface brightness galaxies.
On the Similarity between Cluster and Galactic Stellar Initial Mass Functions
The Astrophysical Journal, 2006
The stellar initial mass functions (IMFs) for the Galactic bulge, the Milky Way, other galaxies, clusters of galaxies, and the integrated stars in the Universe are composites from countless individual IMFs in star clusters and associations where stars form. These galaxy-scale IMFs, reviewed in detail here, are not steeper than the cluster IMFs except in rare cases. This is true even though low mass clusters generally outnumber high mass clusters and the average maximum stellar mass in a cluster scales with the cluster mass. The implication is that the mass distribution function for clusters and associations is a power law with a slope of −2 or shallower. Steeper slopes, even by a few tenths, upset the observed equality between large and small scale IMFs. Such a cluster function is expected from the hierarchical nature of star formation, which also provides independent evidence for the IMF equality when it is applied on sub-cluster scales. We explain these results with analytical expressions and Monte Carlo simulations. Star clusters appear to be the relaxed inner parts of a widespread hierarchy of star formation and cloud structure. They are defined by their own dynamics rather than pre-existing cloud boundaries.
The galaxy-wide IMF of dwarf late-type to massive early-type galaxies
Observational studies are showing that the galaxy-wide stellar initial mass function are top-heavy in galaxies with high star-formation rates (SFRs). Calculating the integrated galactic stellar initial mass function (IGIMF) as a function of the SFR of a galaxy, it follows that galaxies which have or which formed with SFRs > 10 Msol yr^-1 would have a top-heavy IGIMF in excellent consistency with the observations. Consequently and in agreement with observations, elliptical galaxies would have higher M/L ratios as a result of the overabundance of stellar remnants compared to a stellar population that formed with an invariant canonical stellar initial mass function (IMF). For the Milky Way, the IGIMF yields very good agreement with the disk- and the bulge-IMF determinations. Our conclusions are that purely stochastic descriptions of star formation on the scales of a pc and above are falsified. Instead, star formation follows the laws, stated here as axioms, which define the IGIMF th...